1. Field of the Invention
The present invention relates to a so-called coin type or button type non-aqueous electrolyte secondary battery, and more particularly to an improvement in a gasket.
2. Related Background Art
In recent years, the advancement of electronic technology results in an improvement in the performance of electronic apparatuses, size reduction of the same and enhancement in the portability. Therefore, secondary batteries having high energy densities for use in the electronic apparatuses have been required. Hitherto, the secondary batteries for use in the foregoing electronic apparatuses have been nickel-cadmium batteries, nickel-hydrogen batteries or the like. However, the foregoing batteries are unsatisfactory in viewpoints of obtaining those having low battery voltages and high energy densities.
To back up a memory, a coin type secondary battery has been put into practical use, and the battery comprises a negative electrode made of lithium or lithium alloy and a positive electrode made of oxide or hydrosulfide, such as molybdenum, vanadium, titanium or niobium, or spinel-type LiMn2O4 having an improved cycle characteristic of manganese oxide exhibiting a high energy density. However, the foregoing batteries are unsatisfactory in viewpoint of obtaining a very deep cyclic characteristic and load characteristic.
The battery comprising lithium as an active material of the negative electrode has a problem in that lithium deposits in the form of a dendrite because of charging and discharging reactions. It leads to a fact that deposited lithium penetrates a separator, causing internal shortcircuit to occur. The battery, having a structure that the active material of the negative electrode is the lithium alloy, suffers from a problem of deformation of the electrode due to expansion and contraction of the negative electrode which take place during charging and discharging operations.
As disclosed in Japanese Patent Laid-Open No. 62-90863, a non-aqueous electrolyte secondary battery having a structure that a carboneous material is employed as the active material of the negative electrode has been disclosed.
The foregoing non-aqueous electrolyte secondary battery uses doping and de-doping of lithium ions to and from spaces between carbon layers. In this case, the above-mentioned problems can be prevented which arise when lithium or the lithium ions are employed as the active material of the negative electrode and in which lithium in the form of a dendrite is deposited and the electrode is deformed. Therefore, it can be considered that an excellent cycle characteristic can be obtained. In the foregoing case, a material disclosed in Japanese Patent Laid-Open No. 63-135099 or Japanese Patent Laid-Open No. 1-304664 may be employed as the active material of the positive electrode. The disclosed material is composed of a lithium transition composition oxide expressed by, for example, LixMO2 (where M is one or more types of transition metal elements and the relationship 0.05xe2x89xa6xxe2x89xa61.10 is satisfied). In this case, the capacity of the manufactured battery can be enlarged and thus a non-aqueous electrolyte secondary battery having a high energy density can be obtained.
FIG. 1 shows an example of a coin type battery having the foregoing structure that the active material of the negative electrode is a carbon material and the active material of the positive electrode is a lithium transition metal composite oxide.
The coin type battery is manufactured such that a binder is added to powder of the active material of the negative electrode, and then the two materials are mixed uniformly. Then, a thus obtained mixture for forming a negative electrode is pressed so as to be molded. Thus, a pellet 13 for forming the negative electrode is prepared, the negative electrode pellet being then accommodated in a negative electrode cup 11 made of stainless steel. A collector 12 of the negative electrode made of expand metal, such as nickel, is welded to the upper surface of the negative electrode cup 11. A separator 14 containing impregnated non-aqueous electrolyte is disposed on the pellet 13 for forming the negative electrode. On the other hand, a pellet 15 for forming the positive electrode is accommodated in a can 16 of the positive electrode, the pellet 15 being a pellet which has been manufactured by pressing and molding a mixture for forming the positive electrode prepared by adding a conductor and a binder to powder of an active material of the positive electrode and by mixing these materials. The negative electrode cup 11 is engaged to an opening of the can 16 of the positive electrode through a gasket 17. Then, the can 16 of the positive electrode is caulked so that the pellet 13 for forming the negative electrode, the separator 14, electrolyte and the pellet 15 for forming the positive electrode are sealed. Thus, the coin type battery as shown in FIG. 1 can be manufactured.
In the coin type battery having the structure that active material of the negative electrode is the carboneous material and the active material of the positive electrode is the lithium transition metal composite oxide, lithium ions de-doped from the positive electrode are occluded into the material of the negative electrode. Therefore, the battery is designed such that (the diameter of the pellet for forming the negative electrode) greater than (the diameter of the pellet for forming the positive electrode).
However, the conventional coin type battery sometimes encountered undesirable displacement of the position of the pellet for forming the positive electrode. It leads to a fact that the performance of the battery considerably deteriorates. If the pellet for forming the positive electrode is displaced and thus no pellet for forming the negative electrode exists at a position corresponding to the pellet for forming the positive electrode, there is apprehension that lithium is deposited in the form of dendrite on the cup of the negative electrode. As a result, there arises a problem in that charging and discharging ratios are lowered and thus deterioration in the capacity during the charging and discharging cycle becomes-more critical. If the battery is continuously charged, lithium deposited in the form of a dendrite is enlarged to penetrate the separator. In this case, there is apprehension that internal short circuit takes place.
In view of the foregoing, an object of the present invention is to provide a coin type battery non-aqueous electrolyte secondary battery which is capable of reliably preventing displacement of a pellet for forming the positive electrode with respect to the position of the pellet for forming the negative electrode to prevent deposition of lithium onto a cup of the negative electrode in the form of a dendrite so as to realize excellent charging discharging characteristics, continuous charging characteristic and productivity.
To achieve the above-mentioned object, according to the present invention, there is provided a non-aqueous electrolyte secondary battery comprising a pellet for forming a negative electrode accommodated in a can of the negative electrode and a pellet for forming a positive electrode accommodated in a can of the positive electrode which are stacked to face each other through a separator and having a structure that the can of the negative electrode and the can of the positive electrode are caulked through a separator so that the non-aqueous electrolyte secondary battery is sealed, wherein the gasket has an outer annular portion held between the can of the negative electrode and the can of the positive electrode and an inner annular portion facing the internal portion of the battery, and a projection for supporting the pellet for forming .the positive electrode is formed on the inner surface of the inner annular portion at a position opposite to the pellet for forming the positive electrode.
The projection formed on the inner surface of the inner annular portion of the gasket abuts against the outer surface of the pellet for forming the positive electrode so that the position of the pellet for forming the positive electrode is fixed. Therefore, undesirable displacement of the pellet for forming the positive electrode which occurs during assembly of the battery can be prevented.
Since displacement of the pellet for forming the positive electrode can be prevented, deposition of lithium to the negative electrode in the form of a dendrite can be prevented. Thus, charging and discharging characteristics and continuous charging characteristic can be prevented.
Moreover, the battery can easily be assembled and a manufacturing yield can be improved. Therefore, an advantage can be realized in a viewpoint of improving productivity.
Other objects, features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.